Musnad Ahmad 32:208

Yahya narrated to us, from Shu'bah, Qatadah narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Complete the bowing and the prostration, for by Allah, I surely see you from behind me—and he sometimes said: from behind my back—when you bow and when you prostrate."